model: Make Add word interface work

- Word is added but changes are not persistent. That is, it is lost
when application is launched next time. Will fix it later.
model: Make add word changes persistent in letter.properties file

Modify letter.properties file also if a word is added so that next
launch of application can reload it.

Remember that old letter.properties is in bin/ and that needs to
be replaced. Or project needs to be rebuilt for new file to take
affect.

Loading words from database is scheduled for next iteration now.
model: Make delete word work

Now one can delete the word. It updates the letter.properties file
too.

Right now deleted word is not disappearing from list of words in
WPSView. But one can go to alphabet page and letter page and verify
that word got removed.

So some fixing is still neeed.
model: Fix word edit feature.

It is primarily working but there are still some issues. Committing
partial changes now, will fix rest tomorrow.
Merge pull request #26 from CS673/vivek-model-fixes

Vivek model fixes